Dewatering of fine coal with flowing air using low pressure drop systems

Abstract

Successful dewatering and filtration of coal fines remain the major obstacles in preventing the extensive re-use of large reserves of high calorific quality coal fines as an additional energy source in South Africa. The high levels of final moisture in coal fines make it uneconomical to transport, handle and use. The industry is rapidly reaching the limit of current technology of mechanical dewatering; this limit is defined by fundamental coal properties, like amongst others, particle size, porosity and mineral content. This thesis describes research investigating a shift in approach from high vacuum or pressure systems, to high air flow systems. Results from various projects at laboratory scale showed that it was possible to decrease the fine coal filter cake moisture to as low as 15%. This was obtained by allowing air to flow freely through a filter cake, even at ambient temperatures, and replacing the necessity for high applied vacuum levels. There was also an increase in the dewatering rate, as well as a lower breakthrough pressure. Such an approach can utilise existing equipment with minor modifications. Other investigations showed that forced air-drying, both at ambient and elevated temperatures, could be used to overcome this mechanical limit. Again, an increased air flow rate at ambient pressure was used. Using air drying, moisture levels down to zero were possible. These investigations led to the conclusion that increased air flow through a fine coal cake was more advantageous than an increase in the applied vacuum, or a longer dewatering time. This new approach to lowering the final moisture content in coal fines is crucial in any advancement of the use of this largely untapped energy source.